1. Field
Embodiments of the present invention relate to a semiconductor circuit apparatus and a delay difference calculation method.
2. Description of the Related Art
LSIs (Large Scale Integrated circuits) used in an information processing apparatus may have different circuit characteristics due to differences of transistor characteristics. Differences of transistor characteristics result from fluctuations in a semiconductor chip caused by variable factors during manufacturing of the semiconductor chip. Manufacturing fluctuations may occur in a semiconductor chip even if circuits have the same circuit configuration in the same semiconductor chip. While LSIs are normally designed by allowing for manufacturing fluctuations in the semiconductor chip in advance, fluctuations estimated when designed use values determined by investigating characteristics of many transistors by taking many man-hours when a manufacturing process is initiated. Fluctuations in a semiconductor chip increase with finer structures and higher integration of transistors, making the LSI design more difficult.
FIG. 9 illustrates a conventional ring oscillator used for measurement of transistor characteristics of a LSI. A ring oscillator 40 illustrated in FIG. 9A has odd inverters 60 connected in series in a ring shape. Oscillation occurs by logic reversal in odd stages. A ring oscillator 40A illustrated in FIG. 9B has a negative AND (NAND) circuit 50 to control the start and stop of oscillation, instead of a portion of inverters.
If the delay time of a one-stage inverter (delay circuit) is t and the number of stages of inverters is n, an oscillating frequency f of such a ring oscillator is determined as: f=1/(2×t×n). If transistors constituting a ring oscillator have different characteristics, the delay time changes and the oscillating frequency also changes. By observing the oscillating frequency for each of a plurality of ring oscillators, differences in transistor characteristics of each ring oscillator can be observed.
Here, FIG. 10 illustrates an arrangement example of ring oscillators 40 to measure fluctuations inside a LSI. Conventionally, as depicted in FIG. 10, a plurality of identical ring oscillators is mounted and arranged inside a LSI 200 to measure fluctuations inside a chip by comparing oscillating frequencies in units of ring oscillators.
Conventional ring oscillators can determine fluctuations between LSIs by measuring oscillating frequencies of ring oscillators in each LSI. Fluctuations in transistor characteristics inside a LSI can be determined by arranging a plurality of ring oscillators inside the LSI as fluctuations between ring oscillators. However, fluctuations for each transistor or those of transistors by being separated into P-type transistors/N-type transistors cannot be determined. Even if a plurality of ring oscillators is arranged inside a LSI and frequencies are compared, a plurality of transistors constituting a ring oscillator as a whole is compared. Therefore, how transistor characteristics of each transistor are distributed is unknown, and only average characteristics of transistors constituting ring oscillators may be compared. A useful average case is to know an overall trend and in a case of inside a LSI, for example, when it is desired to know whether LSI characteristics change between a perimeter and a central part of the LSI.
As a conventional technique related to the present invention, an oscillating frequency correction circuit enabling auto corrections of an oscillating frequency of a ring oscillator contained in a semiconductor integrated circuit without the need for any external oscillator is disclosed. Also, a voltage control oscillating circuit capable of maintaining the oscillation gain constant to obtain highly stable oscillating frequencies with low jitter is disclosed.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2004-56561
[Patent Document 2] Japanese Patent No. 3779445